Can I get fired for something I posted on facebook?

The fact is that in an "at will" employment work relationship an employer can set the terms and conditions of employment as it deems fit. This includes discipline/discharge for comments made on a social networking site. Accordingly, unless you have a union agreement or employment contract that prohibits your termination for such a posting, or your dimissal violated existing company policy, or your treatment constituted some form of actionable discrimination, your employer did not violate the law. As a general rule employees may be fired due to comments made on a social networking sites.

That having been said, there is an exception regarding terminating an employee for such postings if the griping amounted to what is termed "protected concerted activity". Under federal labor law, employers cannot fire workers for these comments. In some cases it has been found that on-line posts may be no different than workers gathering around a their water cooler to discuss working conditions. The NLRA (National Labor Relations Act) prohibits an employer from punishing an employee for talking about workplace conditions, wages or forming a union (this is true whether or not they are currently union members). The reasoning is that workers should be enabled to communicate amongst themselves in order to decide if in fact a union is necessary.

Based on the limited facts presented it's unclear just exactly what comments were made. Therefore, it cannot be determined if they were protected or not and consequently whether such a termination was wrongful. At this point you can consult directly with an employment law attorney in your area as to your rights and possible remedies.